In recent years, in an optical transmission apparatus such as a terminal station or a relay station, in order to achieve improvement of operability of an operator or automation of setting, a function of automatically measuring and displaying a loss (span loss) of an optical transmission line of a transmission section (span) to another optical transmission apparatus is required.
The span loss measuring function has a function of notifying an operator of a fluctuation in span loss by an external factor during an operation. In a start-up period, the span loss measuring function may be used to check whether a theoretical span loss in a network design phase is matched with an actual span loss or whether an optical level conforms to an input/output specification of the optical transmission apparatus.
Examples of the method of measuring a span loss include a method of arranging photodiodes (PD) in a transmitting station and a receiving station and determining a difference between an optical output level of the transmitting station and an optical input level of the receiving station to measure a span loss.
A measuring method which considers span loss measurement without wavelength dependency of a span loss or signal light is also known. In this measuring method, in a start-up period of an optical communication system, by using amplified spontaneous emission (ASE) light and optical supervisory channel (OSC, monitor control) light, reference span losses based on the ASE light and the OSC light are measured. During an operation of the optical communication system, a difference (fluctuation in span loss) with the OSC reference span loss determined by using the OSC light is added to the ASE reference span loss.
Patent Document 1: Japanese Patent Application Laid-Open No. 2007-251683
In an optical communication system such as a wavelength-division multiplexing system (to be referred to as a WDM system hereinafter), in addition to an optical fiber amplifier such as an erbium doped fiber amplifier (EDFA), a distributed Raman amplifier (DRA) which supplementarily amplifies a WDM light by using a stimulated Raman scattering phenomenon on an optical transmission line may be used.
However, the conventional art described above merely refers to a method of measuring a span loss in an optical communication system in which a DRA is not used.